I'm trying to gain 20 lbs of muscle, and I've researched an read a lot of things.?

Since you said you have P90X, I HIGHLY recommend it. It works great. Just make sure to read the nutrition and fitness guides. I use P90X, and gained muscle, as well I lost some fat. I wasn't fat, but I had some on me. Using P90X, you will be lifting 3 days a week, getting fit with Plyometrics and Kenpo X, as well as doing Yoga. Don't neglect the Yoga. It is important. Not only will you get ripped, but you will get fit, just as long as you do everything right. Good Luck in your decision and have fun getting your old body back!